A seal arrangement for a ship's propeller shaft includes at least one oil-filled seal chamber bounded by seal rings around the shaft, as well as an oil reservoir tank, an oil supply line connecting the tank to the bottom portion of the seal chamber, an oil circulation pump interposed in the supply line between the tank and the seal chamber, and an oil return line communicating from the upper portion of the seal chamber back to the tank. Thereby, oil is continuously circulated through the seal chamber, and thereby flushes out any contaminant particles, air or water that might enter the seal chamber.